1. Field of the Invention
The present invention relates generally to a DC-to-DC converter and to an electric motor drive system using the same.
2. Background Art
A DC-to-DC converter generally transforms (i.e., converts) direct current (“DC”) voltage from a first voltage level to a second voltage level. A DC-to-DC converter that transforms a lower input voltage into a higher output voltage (i.e., steps up the voltage) may be referred to as a “boost” converter. Similarly, a DC-to-DC converter that transforms a higher input voltage into a lower output voltage (i.e., steps down the voltage) may be referred to as a “buck” converter.
Referring to FIG. 1(a), a schematic diagram of a conventional bi-directional DC-to-DC converter 10 is shown. In general, Va, Vb and Vc represent three possible terminal voltages, with Vb being greater than or equal to Va (i.e., Vb≧Va), and Vc being equal to the difference between Vb and Va (i.e., Vc=Vb−Va). The conventional DC-to-DC converter 10 is bi-directional in the sense that any one of the voltages Va, Vb or Vc may be implemented as the input while one or both of the remaining voltages may be implemented as outputs. For example, when Va is implemented as the input and Vb is implemented as an output, the converter 10 is generally a boost converter. Similarly, when Vb is implemented as the input and Va is implemented as an output, the converter 10 is generally a buck converter. Likewise, when Va is implemented as the input and Vc is implemented as an output, the converter 10 is generally a boost-buck converter. Similarly, when Vc is implemented as the input and Va is implemented as an output, the converter 10 is generally a buck-boost converter. The capacitors Ca and Cb of the conventional converter 10 are filter capacitors and the inductor La is a DC choke.
Referring, now, to FIG. 1(b), a schematic diagram of another conventional bi-directional DC-to-DC converter 20 is shown. In general, the converter 20 may be implemented similarly to the converter 10 with the exception that a plurality of capacitors 22, such as Cb and Cc, may be implemented in series across voltage Vb. The use of the plurality of capacitors 22 generally provides a higher voltage rating for the terminal voltage Vb. Accordingly, the converter 20 of FIG. 1(b) may be implemented in systems with high DC voltages, or with capacitors at lower voltage ratings, in comparison with systems using the converter 10.
It is known, however, that parameter variations, such as unmatched capacitances, leakage resistance and the like, between the plurality of capacitors 22 may result in uneven voltage sharing between the capacitors 22. Uneven voltage sharing, in turn, may expose one or more of the capacitors 22 to an over-voltage condition. Conventional attempts to limit occurrences of over-voltage conditions generally require the use of closely matched capacitors or capacitors with higher voltage ratings. The use of such closely matched or higher voltage rated capacitors generally results in an increased cost of manufacturing the controller 20.